This disclosure relates to a process used to manufacture porous fiber mats and proton-conducting composite membranes reinforced with the mats.
Proton Exchange Membrane's (PEM's) life is limited by the swelling and contraction of membranes associated with production water uptake and release during fuel cell operation. Membrane reinforcement has been used in an attempt to enhance the mechanical properties and control the water-swelling characteristics, and therefore, enhancing the membrane's lifetime in PEM fuel cells.
High proton conductivity membranes absorb water and swell to a greater degree. Hence the reinforcement mat property requirements vary with the polymer type. The mat fiber diameter, porosity, fiber connection points, and inter-fiber distance are a few of the parameters that determine the mechanical strength and ability to constrain the swelling.
One example solution for membrane reinforcement is using fibrilliform Polytetrafluoroethylene (PTFE), which is prepared by either extruding perfluorinate sulfonic acid (PFSA) resin (in SO2F form)/PTFE powder mixture with a twin screw extruder into pellets, then hydrolyzing and dispersing the pellets into ethanol-containing solvents, or applying a shearing force to PFSA/PTFE powder dispersion solution to fibrillate PTFE powder into fibrilliform PTFE. The resultant PFSA/fibrilliform PTFE dispersion solution cast into a thin film to create fibrilliform PTFE reinforced ion conducting membrane.
Another example solution for membrane reinforcement includes extruding anhydrous silica containing ethylene/tertrafluoroethylene/C4 terpolymer (ETFE) into a film. The ETFE film was sandwiched between two polyester assist films and subject to simultaneous biaxial orientation to create thin ETFE film. Then, after the assist films were peeled off and anhydrous silica was removed from the thin ETFE film by using hydrogen fluoride, a porous thin ETFE film was obtained, which are filled with a fluoropolymer having a proton conducting group to generate a reinforced proton conducting composite membrane.
Electrospun PVDF nanofiber webs can be used as medical filters, membrane reinforcing materials for genetic separation, and as a support for the electrolyte in batteries. The fiber mat properties, including mechanical and physical, vary with the application. For example, the use of PVDF web (Electrochemica Acta 50 (2004) 339-343) as a polymer electrolyte separator used to support the Li ion electrolyte is not hydration cycled as a hydrogen fuel cell PEM membrane and hence the mechanical strength requirements are not critical.
Nafion impregnated electrospun PVDF composite membranes are discussed by Choi et al., (Journal of Power Sources 180 (2008) 176-171) for direct methanol fuel cell (DMFC) applications. DMFC membranes transport protons while inhibiting methanol crossover. Unlike Hydrogen PEM fuel cell membranes, the DMFC membrane is liquid equilibrated and is not subjected to hydration-based swelling and contraction cycles so Choi et al., did not post process the PVDF fibers to increase the mechanical strength prior to Nafion impregnation. The mechanical properties for the reinforcement, discussed in Choi et al., are driven by ability to process thin membranes to reduce the membrane resistance, but not for durability improvement.
Electrospun polymers for fuel cell applications have also been discussed by Choi et al., (J. Choi, K. M. Lee, R. Wycisk, P. N. Pintauro, and P. T. Mather, J. Electrochem. Soc., 157, B914, 2010). In this art, the authors have electrospun the conducting polymeric material (called charged polymer, like Nafion) and embedded the material in an inert medium (ex. Norland Optical Adhesive) into the Nafion mats (to fill entirely the void space between nanofibers). This is significantly different from the present disclosure where the nonconducting (non-charged) polymer is electrospun and a conducting polymeric material is impregnated into the nonconducting polymer mat.